1. Field of the Invention
This invention relates to a power supply apparatus for use in an ion accelerator which is a discharging device for accelerating ions and more particularly to a Hall thruster serving as an electric propulsion device mounted on an artificial satellite.
2. Description of the Related Art
A Hall thruster ionizes gas introduced from one end of an annular-shaped discharging channel, accelerates the ionized gas and ejects the gas from the other end of the discharging channel. A thrust of a Hall thruster can be provided due to the reaction of the ions thus produced. A magnetic flux is radially formed in the annular-shaped discharging channel. The Hall effect by the magnetic flux allows electrons to circumferentially drift in the annular-shaped discharging channel so that the movement of the electrons is suppressed in an axial direction. Thus, only the ions can be effectively accelerated (for example, see JP-A-2002-517661).
One of the problems in stably operating the Hall thruster is occurrence of a discharge oscillation phenomenon. The discharge oscillation phenomenon includes various kinds of oscillation phenomena, which include the discharge oscillation phenomenon at the lowest frequency called “ionization oscillation.” In this discharge oscillation phenomenon, an oscillation is generated in the current waveform of the anode current, at a frequency of about 10 kHz, thereby having serious effects on stability, reliability and endurance of a system incorporating the Hall thruster. In order to obviate such inconvenience, a control technique for suppressing this discharge oscillation phenomenon has been demanded (for example, see Kyouichi Kuriki and Yoshihiro Arakawa, “Introduction to Electric Propulsion Rockets,” University of Tokyo Press, pp. 152-154, 2003, Japan). Further, a condition for generating the discharge oscillation phenomenon in the Hall thruster has been previously formulated using a relatively simpler model (for example, see N. Yamamoto, K. Komurasaki and Y. Arakawa, “Discharge Current Oscillation in Hall Thrusters,” Journal of Propulsion and Power, Vol. 21, No. 5, pp. 870-876, 2005).
In a conventional power supply apparatus, when a load begins to exhibit an unstable behavior due to changes in an anode current, an anode current signal is fed back to a power supply control unit to suppress changes in the anode current, thereby suppressing the discharge oscillation phenomenon (for example, see JP-A-2005-282403).
In the conventional power supply apparatus, when the anode current varies, the anode current signal is fed back to the power supply unit, thereby suppressing the anode current. However, in such a technique of detecting that the anode current has begun to vary, since the discharge oscillation phenomenon is not theoretically suppressed, it is difficult to essentially enhance the stability of the Hall thruster. Further, the discharge oscillation phenomenon occurs at the frequency of e.g. 10 kHz. For this reason, when it is intended to suppress the oscillation by the feed-back to the power supply control unit, a very high speed control system is required. If the control system cannot deal with a high speed response, stable control cannot be realized, and the oscillation phenomenon occurs between the power supply apparatus and the control system so that unstability of the Hall thruster may be promoted. Moreover, when the condition for driving the Hall thruster is optimized while aiming at the effect different from stability of the current in a specific operating condition, disadvantageously, the Hall thruster cannot be stably controlled.